1. Field of the Invention
The present invention relates to a drive power distribution control method and device for a four-wheel drive vehicle. It also relates to a four-wheel drive vehicle having the drive power distribution control device.
2. Discussion of the Related Art
Heretofore, there has been known a drive power distribution control device for a four-wheel vehicle, wherein the drive power transmission rate of a drive power transmission device is variably controlled in dependence upon a vehicle speed and an acceleration manipulation amount (e.g., a throttle opening degree in the case of a gasoline engine car) so as to variably control the drive power distribution rate between front and rear wheels. More specifically, a drive power (transmission torque) depending on the vehicle speed and the acceleration manipulation amount is obtained by reference to a predetermined torque characteristic map, and the friction engagement force of an electromagnetic clutch constituting the drive power transmission device for the four-wheel drive vehicle is controlled so that the torque so obtained can be transmitted to the front wheels or the rear wheels. The torque characteristic map is a table map of the type that a transmission torque can be extracted by designating the vehicle speed and the acceleration manipulation amount as parameters and is prepared in advance by experiments using a vehicle model or by a well-know theoretical calculation processing.
However, the foregoing known drive power distribution control device for the four-wheel drive vehicle involves the following problems. For example, in a vehicle with an automatic transmission (hereafter referred to as xe2x80x9cAT vehiclexe2x80x9d), the vehicle in an idling state with the transmission being selected to D-range is advanced due to a so-called creep phenomenon even when the driver does not make the acceleration manipulation (i.e., stepping-on manipulation of an accel pedal). This is because the drive power of an engine being in the idling state is transmitted to the drive wheels through a torque converter of the AT (automatic transmission).
For the reason mentioned above, even during the creep motion or a low speed traveling, the drive wheels (i.e., the front wheels in a vehicle of the front-drive basis) tend to slip on an excessively small xe2x80x9cxcexcxe2x80x9d road (i.e., a slippery road such as an xe2x80x9ceisbahnxe2x80x9d or the like). For example, where the vehicle stands stopped on an ascending road with the xe2x80x9ceisbahnxe2x80x9d, the front wheels happen to slip at the moment that the driver shifts to the D-range. In this case, the problem can be solved by increasing the drive power distributed to the driven wheels (e.g., rear wheels) in dependence on the rotational speed difference between the front and rear wheels.
However, it is often the case that the drive power distributed to the driven wheels is to be restrained in order to avoid a tight-corner braking phenomenon which occurs at the time of, for instance, garaging or the like (namely, the phenomenon in which a braking torque is generated against the front wheels due to the difference in the average turning radius between the front and rear wheels in the case for example that much more drive power is distributed to the rear wheels during a turning motion). This does not allow to unconditionally increase the drive power to be distributed to the rear wheels (driven wheels) in dependence on the rotational speed difference between the front and rear wheels. Accordingly, it has been difficult to restrain the front wheels (drive wheels) from slipping at the time of starting on the excessively small xe2x80x9cxcexcxe2x80x9d road.
Further, the known four-wheel drive vehicle involves another problem concerning a quick starting which can be done with the engine being kept rotated at a high speed. If the front wheels are brought into connection with the rear wheels at such a quick starting, it often occurs that the passengers suffer a shock (torque shock). Such a torque shock has been verified to be large when the vehicle is started quickly with the drive power output from the engine being maintained at a high power.
In addition, another drive power distribution control device for a four-wheel vehicle of a similar type has also been known as described in Japanese unexamined, published patent application No. 2002-206566. In this known drive power distribution control device, when the drive mode is set in AUTO mode, the opening degree of a throttle valve, a vehicle speed and a rotational difference between the rotational speeds of front and rear wheels are extracted from two torque maps based on signals from sensors and are made reference thereto to extract a first transmission torque depending on the throttle opening degree, a second transmission torque depending on the front-rear rotational difference and first and second gains depending on the vehicle speed. A command transmission torque to be distributed to the rear wheels is calculated by summing up the product of the first transmission torque and the first gain and the product of the second transmission torque and the second gain. And, in the know device, the torque maps are designed so that first and second transmission torques are increased with the respective increases in the throttle opening degree and the front-rear rotational difference, while the first and second gains are decreased with increase in the vehicle speed; i.e., the first and second gains are set larger when the vehicle speed is low.
However, the prior art of the Japanese patent application does not describe how to control the command transmission torque to be distributed to the rear wheels at the starting of the vehicle in connection with the state or manner in which the vehicle is beginning to start, such as for example the state of a road surface on which the vehicle is beginning to start or the manner of beginning to start the vehicle.
Accordingly, it is a primary object of the present invention to provide an improved torque distribution control device for a four-wheel drive vehicle capable of restraining drive wheels from skidding or slipping at the starting of the vehicle.
Another object of the present invention is to provide a four-wheel drive vehicle having an improved torque distribution control device capable of preventing or restraining a torque shock from being generated even when the vehicle is brought into the four-wheel drive state with an engine being in the state of a high power output.
Still another object of the present invention is to provide an improved torque distribution control device for a four-wheel drive vehicle capable of controlling the drive torque to be transmitted to rear wheels so that the slips or skids of both front and rear wheels can be restrained to the least.
Briefly, according to a general inventive concept of the present invention, there is provided a drive power distribution control method and device for a four-wheel drive vehicle for variably controlling the amount of drive power distributed to a driven wheel axle by variably controlling the power transmission rate of a drive power transmission device based on signals from various sensing means including at least vehicle speed sensing means for detecting the vehicle speed. The drive power distribution control method and device comprises a traveling state judgment step and means for judging whether the vehicle is beginning to start or not and also judging the state or manner in which the vehicle is beginning to start. The drive power distribution control method and device further comprises a transmission torque control step and means operable when it is judged based on a vehicle speed signal that the vehicle is beginning to start, for controlling the drive torque transmitted to the driven wheels in dependence on the state or manner judged by the traveling state judgment step and means.
With this configuration, the starting of the vehicle is discriminated from an ordinary four-wheel drive traveling state, and when it is detected that the vehicle is beginning to start, the drive torque transmitted to the driven wheels is controlled in dependence on the state or manner in which the vehicle is beginning to start. For example, the vehicle may be beginning to start in a state such as a slippery road whose surface friction coefficient is excessively small, or in a manner such as a quick starting. Therefore, when the vehicle starts, a drive torque appropriate to the state or manner in which the vehicle is beginning to start is transmitted to the drive wheels, so that slips or skids of either of the front and rear wheels can be restrained to the least.
In one specific aspect of the present invention, there is provided a drive power distribution control method and device for a four-wheel drive vehicle for variably controlling the amount of a drive power distributed to a driven wheel axle by variably controlling the power transmission rate of a drive power transmission device based on a vehicle speed obtained from a vehicle speed sensing step and means and an acceleration manipulation amount obtained from an acceleration manipulation amount detection step and means. The control method and device comprises a traveling state judgment step and means for judging whether the vehicle is beginning to start or not, and a control step and means for controlling the power transmission rate of the drive power transmission device to make the amount of the drive power distributed to the driven wheel axle larger than an ordinary value for an ordinary four-wheel drive traveling when it is judged by the traveling state judgment step and means that the vehicle is beginning to start.
With this configuration, the drive power transmission rate of the drive power transmission device is variably controlled based on the vehicle speed obtained from the vehicle speed detection step and means and the acceleration manipulation amount obtained from the acceleration manipulation step and means and thus, the drive power to be distributed to the driven wheels is controlled variably. When the traveling state of the vehicle is at the starting, the transmission rate of the drive power transmission device is controlled in such a manner that much more drive power than an ordinary value is transmitted to the driven wheels. As a result, the drive wheels can be restrained from slipping at the time of starting.
In another specific aspect of the present invention, there is provided a four-wheel drive vehicle having a primary drive wheel axle connected to drive wheels; a secondary drive wheel axle connected to driven wheels; drive means for driving said primary drive wheel axle; and a drive power transmission device for transmitting a drive power from said drive means to said secondary drive wheel axle. There is further provided a control method for the four-wheel drive vehicle of the aforementioned construction. The control method and the four-wheel drive vehicle further comprise a detection step and means for detecting whether a quick starting has taken place or not of said vehicle, based on a predetermined high drive power output from the drive means; and a control step and means for reducing the drive power transmitted from the drive power transmission device to the secondary drive wheel axle when the quick stating has taken place, by a predetermined amount than that which the drive power transmission device transmits to said secondary drive wheel axle when the vehicle is in an ordinary four-wheel drive traveling.
With this configuration, when the quick stating has taken place with the drive means outputting the predetermined high drive power output, the drive power transmitted from the drive power transmission device to the secondary drive wheel axle is reduced by the predetermined amount than an ordinary drive power given to the secondary drive wheel axle in the ordinary four-wheel drive traveling. Thus, the toque shock which would otherwise occur when the quick starting of the vehicle is performed can be prevented from being generated or restrained to the least.
In a further specific aspect of the present invention, there is provided a control method and device for a drive power transmission device of a vehicle of the type wherein one drive axle and the other drive axle are connected by the drive power transmission device in dependence upon a wheel speed difference between drive wheels driven by the drive power of said one drive axle and driven wheels driven by the drive power of said other drive axle so as to transmit the drive power from said one drive axle to said other drive axle. The control method and device comprises a vehicle starting detection step and means for detecting whether the vehicle is beginning to start or not; a slip amount calculation step and means for calculating a slip amount based on the wheel speed difference between the drive wheels rotated by the drive power of said one drive axle and the driven wheels rotated by the drive power of said other drive axle; and a connection degree determination step and means responsive to the slip amount calculated by the slip amount calculation step and means for determining the connection degree by the drive power transmission device of said one drive axle with said other drive axle when it is detected that the vehicle is beginning to start.
With this configuration, when it is detected that the vehicle is beginning to start, a slip amount is detected from a wheel speed difference between the drive wheels rotated by one drive axle and the drive wheels rotated by the other drive axle, and the connection degree of the drive power transmission device of said one drive axle with said other drive axle is controlled in dependence on the detected slip amount. Thus, the slip of the vehicle can be restrained in adaptation for the state of a road surface on which the vehicle is beginning to start, so that the performance of the vehicle at the time of starting can be enhanced where the road is of a small xe2x80x9cxcexcxe2x80x9d (friction coefficient) surface or bumpy. In addition, where the vehicle travels with little slip on a large xe2x80x9cxcexcxe2x80x9d surface road, the torque distribution to the driven wheel can be made small, so that the fuel consumption performed can be improved.